Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code-division multiple access (CDMA) systems, time-division multiple access (TDMA) systems, frequency-division multiple access (FDMA) systems, and orthogonal frequency-division multiple access (OFDMA) systems.
By way of example, a wireless multiple-access communication system may include a number of base stations, each simultaneously supporting communication for multiple user equipments (UEs; e.g., mobile devices). A base station may communicate with UEs on downlink channels (e.g., for transmissions from a base station to a UE) and uplink channels (e.g., for transmissions from a UE to a base station).
Some modes of communication may enable communication with a UE over different radio frequency spectrum bands (e.g., a licensed radio frequency spectrum band and/or an unlicensed radio frequency spectrum band). With increasing data traffic in cellular networks, the offloading of at least some data traffic from a licensed radio frequency spectrum band to an unlicensed radio frequency spectrum band may provide a cellular operator with opportunities for enhanced data transmission capacity. In other examples, an unlicensed radio frequency spectrum band may be used in a standalone mode where access to a licensed radio frequency spectrum band is not available.
Prior to transmitting data over an unlicensed radio frequency spectrum band, a transmitting apparatus may, in some examples, perform a clear channel assessment (CCA) procedure to gain access to the unlicensed radio frequency spectrum band. A CCA procedure may determine whether a particular channel of the unlicensed radio frequency spectrum band is available. When it is determined that the channel of the unlicensed radio frequency spectrum band is not available (e.g., because another device is already using the channel of the unlicensed radio frequency spectrum band), a CCA may be performed for the channel of the unlicensed radio frequency spectrum band again at a later time.
Because a base station may contend for access to an unlicensed radio frequency spectrum band, there is a chance that the base station may be unable to transmit a periodic signal such as a positioning reference signal (PRS) at a predetermined time period interval. When one or more base stations fail to transmit a PRS, a UE loses one or more opportunities to make PRS measurements, and it may not be possible to determine an accurate position of the UE from its PRS measurements.